Add composer instructions

Development.md

@@ -81,6 +81,16 @@ public function createComment($pasteid, $parentid, $commentid, $comment)
 }
 ```
 
+## Composer
+
+If you need to execute composer, because you e.g. want to include or update the dependencies of PrivateBin, please run this command:
+
+```shell
+composer install --no-dev --optimize-autoloader
+```
+
+Note that the `.gitignore` will exclude all non-PHP files in the vendor dir when you create a commit.
+
 ## Subresource Integrity for Javascript resources
 
 Because we implemented [Subresource Integrity (SRI)](https://www.w3.org/TR/SRI/) the SRI hashes in the templates need to be regenerated after each change of a javascript file. To make this easy, we implemented a mechanism for this into the unit testing bootstrap process. After changing any javascript file, follow this process: